Technology for the Production of Clean, Thin-wall, Machinable Gray and Ductile Iron
All high production foundries experience problems with casting machinability. Some castings are scrapped immediately while others make it to the machine tool line where they cause inordinate tool wear and sometimes break the tools. Premature tool wear forces the line to be slowed or shut down for tool replacement. Not only must new tools or tool inserts be used, but there are high costs associated with line downtime. It has historically been difficult and usually impossible to assign causes to the specific tool wear problem because there was limited data about relations between foundry processes and tool life. That deficiency is being eliminated in this project.
The focus of the Phase I study was to develop a consistent method for evaluating the machinability of gray and ductile iron. Test castings were produced in participating foundries, and "hard-to-machine" castings were solicited from both foundries and machine shops. Accomplishments from that first effort include the following:
- Microcarbides present in irons were found to dominate the machinability of iron. Pearlitic irons considered to have "acceptable" machinability were found to contain from 8.9 to 10.5% by weight microcarbides. The weight fraction microcarbides in the iron is influenced by carbide forming element concentrations, the presence of elements that retard carbon diffusion, and the cooling rate from the eutectic through the eutectoid temperature range. The tool wear rate increased when machining at higher surface speeds and when machining irons containing higher weight percentages of microcarbides.
- Inoculation trials indicated that the amount of inoculant added was found to have a significant effect on tool life. Reducing the addition of foundry-grade calcium and aluminum bearing 75% FeSi inoculant from 0.5% to 0.2% increased tool life by about 100%.
- Exploratory studies showed that torque and feed forces were found to correlate with machinability.
A large body of data on iron processing, properties, and machinability was developed to provide baseline information relating machinability to microstructural characteristics.
The primary focus of current research is to determine how the machinability of gray and ductile iron castings can be improved to support the development of thin walled gray and ductile iron castings for use in the ground transportation industry. As noted, excessive microcarbides were found to be a dominate factor degrading machinability of iron castings. One of the principle goals of the current program is to determine how the occurrence of microcarbides can be controlled by normal foundry processing changes (rather than heat treatment).
Castings submitted in Phase II were examined and compared to the baseline data. The baseline data serves as a reference point for assessing the causes of the difference. Batches of iron were produced and evaluated to study the effects of processing on machinability. It was found in duplicate experiments that higher silicon concentrations (with other conditions held constant) improved machinability. This information can be applied immediately to improve casting machinability. It was also found that cleaning practice at the foundry affects machinability.
Excessive microcarbides have been found in prior research to be a dominate factor degrading machinability of iron castings. It was demonstrated in the last quarter that the occurrence of microcarbides can be controlled by carefully controlling the tin and chromium concentrations and by controlling the shakeout temperature of the iron.
A second abrasive phase, not reported in the published literature and very detrimental to machinability, was found in castings submitted early in 1999, and many more examples of the inclusions were found in subsequent quarters. The cause of the troublesome phase lies in either incomplete dissolution of silicon carbide added to the melting furnace or incomplete dissolution if inoculating additions made to the metal before pouring. Plant experiments are underway to define the source and cure of the problem. It appears that machinability can be improved by a factor of about five (500%) by eliminating this phase.
The primary focus of the current phase will be to continue to identify and determine how the occurrence of microcarbides, silicides, and other objectionable phases can be controlled within the normal foundry process (rather than by heat treatment). Alloy combinations will be explored that will maintain strength while improving machinability. Machining operations will be extended from drilling to include turning, and data will be obtained with higher performance tools including carbides and ceramics. In addition, properties will be measured on selected classes of irons to provide data for linear elastic stress codes that can be used to design castings for reduced mass.